In order to enable long-hour use of portable electronic devices such as laptop personal computers and cellular phones without charging, use of fuel cells as power sources and chargers of these portable electronic devices has been attempted. The fuel cell has a feature that it is capable of generating electricity only by being supplied with a fuel and air and is capable of continuously generating electricity for long hours by being replenished with the fuel. Therefore, the fuel cell can be the to be an extremely advantageous system as a power source and a charger of portable electronic devices if it can be made compact.
A direct methanol fuel cell (DMFC) is expected to be promising power source and charger of portable electronic devices because it can be made compact and its fuel can be handled easily. As a method of supplying a liquid fuel in the DMFC, there have been known an active method such as a gas supply type and a liquid supply type, and a passive method such as an internal vaporization type in which a liquid fuel in a fuel storage unit is vaporized inside the cell to be supplied to a fuel electrode.
The passive method such as the internal vaporization type is especially advantageous to miniaturization of the DMFC. There has been proposed a passive-type DMFC which is structured such that, for example, a membrane electrode assembly (MEA) having a fuel electrode, an electrolyte membrane, and an air electrode is disposed on a fuel storage unit composed of a box-shaped container (see, for example, patent reference 1). It has also been under consideration to connect an electromotive unit of the DMFC and the fuel storage unit via a flow path (see patent references 2 to 3).
When a methanol fuel with a high concentration or the like introduced from the fuel storage unit directly or via the flow path is vaporized and supplied to the fuel electrode, it is necessary to discharge gas components such as carbon gas, water vapor and the like generated due to cell reaction to the outside of the system while trapping the gasified fuel on the fuel electrode side of the MEA. In this regard, it has been considered in the DMFC of the conventional passive-type and the like to provide a gas vent hole in a side surface of the container on the fuel electrode side so as to release the gas components to the outside of the system.
However, when the gas vent hole is provided in the side surface of the container of the DMFC, the generated gas components escape from a peripheral portion of the MEA so that the gas components generated near the middle of the MEA cannot be sufficiently removed. This causes a problem of instability of electric generation characteristics of the DMFC. Further, the temperature is lower at the peripheral portion of the MEA than near the middle thereof and apt to cause clogging of the gas vent hole because of condensation of water vapor. This brings about a problem of failing to obtain stable output characteristics over time in the DMFC.    Patent Reference 1: WO 2005/112172 A1    Patent Reference 2: JP-A 2005-518646 (KOHYO)    Patent Reference 3: JP-A 2006-085952 (KOKAI)